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import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Home(),
    );
  }
}

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _HomeState();
  }
}

class _HomeState extends State<Home> {
  static ScrollController scrollController = new ScrollController();
  int _currentIndex = 0;
  final List<Widget> _children = [
    ListPage(),
    BlankPage(),
    BlankPage()
  ];
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Hello'),
      ),
      body: _children[_currentIndex], // 当_currentIndex不变时，它时不会重绘页面的！
      bottomNavigationBar: BottomNavigationBar(
        type: BottomNavigationBarType.fixed,
        onTap: onTabTapped, // new
        currentIndex: _currentIndex, // new
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            title: Text('Home'),
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.search),
            title: Text('Search'),
          ),
          BottomNavigationBarItem(
              icon: Icon(Icons.person),
              title: Text('Me')
          )
        ],
      ),
    );
  }
  void onTabTapped(int index) {
    setState(() {
      if (_currentIndex == index) {
        //how to controll the scrollConroller of ListPage?
      }
      _currentIndex = index;
    });
  }
}

class ListPage extends StatefulWidget{
  @override
  State<ListPage> createState() {
    return ListPageState();
  }
}

class ListPageState extends State<ListPage> {
  ScrollController _scrollController = new ScrollController();
  @override
  Widget build(BuildContext context) {
    return Stack(
      children: <Widget>[
        Container(
          child: ListView.builder(
            itemCount: 100,
            itemBuilder: (BuildContext context, int index){
              return Text("$index");
            },
            controller: _scrollController,
          ),
        ),
        Positioned(
          right: 15,
          bottom: 15,
          child: FloatingActionButton( //ListPage页内的控件可以控制_scrollController
            child: Icon(Icons.arrow_upward),
            onPressed: () {
              _scrollController.animateTo(0.0, duration: Duration(milliseconds: 300), curve: Curves.easeOut);
            },
          ),
        )
      ],
    );
  }
}

class BlankPage extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    return Center(
        child: Text("Hello"),
    );
  }
}